The present invention relates to a braking pressure control device for a motor vehicle with hydraulic multi-circuit brake systems with static brake circuits, the motor vehicle having a master cylinder as a braking device with separate pressure outlets associated with each of said brake circuits; a pneumatic braking force amplifier coupled to said master cylinder and a brake pedal coupled to said pneumatic braking force amplifier. The brake pedal actuates the master cylinder through the amplifier, this amplifier having a drive chamber movably delimited by a piston between a permanent low-pressure chamber maintained at a low pressure, whose charging with a pressure monotonically correlated with pressure on the brake pedal is controllable by which a deliberate brake application occurs. An auxiliary force valve is coupled to the amplifier and switchably causes exposure of the low-pressure chamber to a pressure equal to or greater than atmospheric pressure, this exposure causing a high actuating force sufficient for a full brake application to be deployed. An electronic control unit having an output signal coupled to the auxiliary force control valve controls the triggering of the switching of the auxiliary force control valve. The electronic control unit generates an output signal when the speed or the rate of change of the force with which the driver actuates the brake pedal exceeds a threshold value. An antilock brake system (ABS) is coupled to and acts on the individual brake circuits by feedback. The brake systems have inlet and outlet valves associated individually with the wheel brakes and are electrically controllable. Feedback pumps are individually associated with the brake circuits, these pumps being drivable electrically and having a high output pressure level. The pumps, in a pressure-reduction phase of antilock brake regulation, recycle into a main brake line of the respective brake circuit the brake fluid that has been released into a return line of the respective brake circuit.
A braking pressure control device of the above-described type according to related U.S. patent application Ser. No. 08/033,757, filed on Mar. 17, 1993, now U.S. Pat. No. 5,350,225 is intended for a vehicle with static brake circuits, each circuit having associated with it one output of a master cylinder provided as a braking device. This master cylinder is operable by a brake pedal through a pneumatic braking force amplifier. The amplifier has a drive chamber movably delimited by a piston from a low-pressure chamber kept permanently at lower pressure, by whose exposure via pedal control to a pressure monotonically correlated with the pedal force, the force amplification with which a deliberate brake application takes place can be controlled. By exposure to atmospheric pressure or to a higher pressure, produced by the switching of a solenoid valve, an actuating force that is high and sufficient for a full brake application acts on the master cylinder, can be deployed. Switching of the solenoid valve for this purpose can be triggered by an output signal from an electronic control unit that generates the output signal when the speed .phi. or the rate of change K of the force K with which the driver actuates the brake pedal exceeds a threshold value .phi..sub.S or K.sub.S in this respect. The vehicle is also equipped with an antilock brake system (ABS) designed to ensure dynamically stable behavior of the vehicle during a brake application, and acting on the individual brake circuits using the feedback principle. This system comprises the intake and exhaust valves associated with the individual wheel brakes and is controllable electrically. Feedback pumps are assigned individually to the brake circuits and are electrically drivable and designed for a high output pressure level. By means of the feedback pumps, in a pressure reduction phase of the antilock regulation, brake fluid released into a return line from the respective brake circuit can be fed back into its main brake line that branches off one of the pressure outlets of the brake system.
The purpose of the braking pressure control device according to the related patent application is to support automatically the driver's desire, as determined from the manner of actuation of the brake pedal, for greater vehicle deceleration in such a manner that even while the brake pedal is traveling the distance that is usually available, a relatively high braking pressure is coupled into the wheel brakes in order to activate the brake system as soon as possible. This rapid response of the brake system along with a correspondingly high increase in vehicle deceleration is also achieved, with the control point of the braking force amplifier being reached in only a short time. This "fast" response behavior of the brake system, in cases that are not uncommon, also encourages the driver not to increase further the force with which he is actuating the brake pedal, since after the control point of the braking force amplifier is reached and the brake pedal, after initially being very easy to depress, suddenly exhibits a different, harder reaction, the driver has the impression that he has exhausted the braking force and vehicle deceleration available to him. The result of such behavior on the part of the driver is that a significant portion of the actuating force in the master cylinder that can be converted into braking pressure, namely the amount that the driver could exert, is wasted as it were. This has the additional consequence that, especially in brake applications made on a dry road with good traction, the maximum possible vehicle deceleration that would result if the braking force were to be increased until the antilock brake system of the vehicle responded, is not utilized.
An object of the invention is to improve a braking pressure control device of the initially described type so that even in the event of an automatically controlled full brake application, the maximum possible vehicle deceleration values can be reliably utilized.
This and other objects are achieved by the present invention which provides in a brake control device of the initially mentioned type a separate electrically controllable switching valve for each of the brake circuits, said valve being switchable from a functional position connecting the main brake line of the respective brake circuit with the respective pressure outlet of the braking device, into a functional position shutting off the respective pressure outlet from the respective main brake line. An electrically controllable precharging control valve is provided that is switchable from a functional position shutting off the respective pressure outlet of the braking device from a low-pressure inlet of a return pump of the respective brake circuit, into a functional position connecting the respective pressure outlet with the low-pressure inlet of the respective return pump. The electronic control unit includes sensor inputs that receive sensor output signals that contain information regarding at least one of the actuation of the brake pedal and the dynamic behavior of the motor vehicle, the electronic control unit generating signals required for automatic operation of the brake system at a high braking pressure level, for controlling the switching valves, the precharging control valves, the return pumps, and the inlet and outlet valves of the antilock brake system, and the auxiliary force control valve.
According to the present invention, an electrically controllable switching valve is provided for each of the brake circuits, said valve being switchable from a functional position (its basic position) in which it connects the main brake line of the respective brake circuit with the pressure output from the master cylinder associated therewith, into a functional position that shuts off the pressure output from the main brake line, as well as an electrically controllable precharging control valve that is switchable from a functional position (its basic position) in which it shuts off the respective pressure output of the master cylinder from the low-pressure inlet of the return pump of the respective brake circuit, into a functional position that links the respective pressure output with the low-pressure inlet of the respective feedback pump, with the output pressure of the master cylinder, actuated for example by venting the drive chamber of the pneumatic braking force amplifier, being conducted as a precharging pressure to the return pumps of the antilock brake system, which can now feed brake fluid at a high output pressure level into the main brake lines of the two brake circuits, and an electronic control unit is provided that generates information on the nature of the actuation of the brake pedal and possibly information on the dynamic behavior of the vehicle, and generates the signals required for automatic operation of the brake system at a high braking pressure level for controlling the switching valves, the precharging control valves, the return pumps, and the inlet and outlet valves of the antilock brake system.
In the operating state of the brake system of the present invention in which the return pumps are utilized as pressure sources individually associated with the brake circuits, and the braking device including its braking force amplifier is utilized as their precharging pump, very high pressures can be coupled into the individual wheel brakes, which can also be adjusted by individual pulsed control of the inlet and outlet valves of the individual wheel brakes to provide individually different values. The brake system of the invention is therefore suitable for a driving-dynamics regulating operation which, depending on the situation, can require the activation of one or more wheel brakes at different pressure levels. This is especially true in certain embodiments that provide pressure sensors that are individually associated with the wheel brakes of the vehicle, whose pressure-proportional output signals can be processed by the electronic control device as actual value signals.
In addition, the brake system provided in certain embodiments is especially advantageous for driving-dynamic braking pressure regulation, and also for automatic control of full brake application operation of the brake system, since, in addition to monitoring the brake pedal position by means of a pedal position sensor, it permits additional determination of the brake-actuating behavior of the driver.
Certain embodiments of the invention provide a force-sensor device suitable for determining the driver's wishes with regard to the initiation and termination of an automatically controlled full brake application, something that is possible for example with the aid of strain gauges or piezoelectric force-sensing elements which correspond to the available prior art, and can be incorporated into the brake system without significant design modifications in the braking force amplifier or the master cylinder.
In certain advantageous embodiments, a functionally corresponding sensor arrangement is provided by using a simple microswitch that can be integrated without difficulty into the braking force amplifier.
Certain embodiments of an electronic control unit provided in the present invention permit an especially reliable and sensitive determination of the driver's wishes.